Silicon nitride (Si3N4) nanowires and nanobelts have been successfully prepared via direct crystallization of amorphous Si3N4 nanoparticles under high temperature in a N-2 flow. The products were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution electron microscopy, and electron diffraction. They are pure alpha-phase hexagonal single-crystal structures. The nanowires are long and smooth; nanobelts are long and twisted. In our samples, there exist some special nanostructures, such as wire-inserted hexagonal nanosheet and hollow-chain-shaped structure. The different growth modes were understood upon the observations and characterization of those microstructures. The solid-liquid-solid growth mechanism is also discussed.